Magnetic compositions



Patented Nov. 13, 1951 MAGNETIC COMPOSITIONS Henry L. Crowley, South Orange, N. J., assignor to Henry L. Crowley & Company, Inc., a corporation of New Jersey No Drawing. Application February 18, 1950, Serial No. 145,085

This invention relates to magnetic compositions and articles formed therefrom, and has for its object the provision of new and useful magnetic compositions and articles formed therefrom, as new articles of commerce, and a method of making the same. The magnetic compositions of the invention and articles produced therefrom possess useful magnetic and electrical properties, and have a principal use as cores in inductances, transformers, deflection coils and similar devices employed in the communication and other electrical arts.

Magnetic cores have heretofore been made of finely divided iron or a highly magnetic ferroalloy mixed with a small amount of an insulating and bonding agent such as shellac. The fundamental characteristic of the metallic core is subdivision of the metal particles and their insulation by films of the insulating agent to reduce the gross electrical conductivity of the mass of the core, thus reducing eddy current losses when the core is subjected to alternating magnetization. This improvement is obtained, however, only by sacrifice of apparent magnetic permeability. Since the core is non-homogeneous, miniature poles appear at each surface of discontinuity, and exert a demagnetizing effect opposite to the magnetizing field. The smaller the size of the metal particles, the greater is the demagnetizing effect. The effect is so pronounced that despite the high permeability of the iron or ferro-alloy, measuring thousands of units in the massive state, the apparent permeability of conventional cores may be of the order of 5 to 25 units, and increase to above about 100 units cannot be achieved without entailing excessive eddy current losses.

More recently, non-metallic materials having relatively high permeabilities have been developed for use as magnetic cores. Among these are compounds of ferric oxide with one or more divalent oxides of metals other than iron (commonly called ferrites). In preparing these materials, the mixture of ferric oxide and the other metal oxide or oxides is sintered at an elevated temperature for a relatively long period in an oxygen atmosphere, since loss of oxygen during sintering adversely affects the desired properties of the finished material. The ferrites are characterized by very low Curie points which limit their application where substantial temperature rises may be encountered. But manufacturing difliculties present a most serious drawback to wider use of the ferrites. The component oxides begin to react at temperatures of GOO-800 C.,

24 Claims. (Cl. 252-625) and the resulting crystalline ferrites, while magnetically soft in certain compositions of admixture, are relatively infusible, in consequence of which homogenizing is extremely slow even at the highest temperature that can be tolerated short of that at which the ferric oxide constituent begins to dissociate.

The composition of the invention may be briefly described as a crypto-crystalline ferritic ferrite, whose essential nature is that of magnetite modified for the improvement of its magnetic and electrical properties. Its ferritic or metallic iron content is produced internally or in situ by the reduction of ferric oxide to ferrous oxide, and the spontaneous decomposition of the resulting ferrous oxide, in consequence of which the iron-is initially distributed in an extremely fine state of subdivision (a far finer state of dissemination than is possible in powdered iron). Its ferrite content is essentially a mixed ferrite in which modifiers of magnetic and electrical properties have replaced ferrous oxide in the ferrous ferrite (magnetite) formed initially by the spontaneous decomposition of ferrous oxide. Evidence of the spontaneous decomposition of ferrous oxide can only be obtained by magnetic measurement or special chemical tests, and hence the composition is crypto-crystalline. The foregoing and other characteristics of the composition will be better understood from the following discussion of its preparation by the method of the invention.

Briefly, the method of the invention in its present preferred aspect involves the following steps:

1. Intimately mixing ferric oxide with suitable quantities of modifying divalent metal oxides, and preferably including a small amount of a carbonaceous material inthe resulting mixture,

2. Pre-firing or heat-treating the initial oxide mixture in a reducing environment capable of reducing a large part of the ferric oxide (F6203) to ferrous oxide (FeO) and, if desired, capable of further reducing part of the ferrous oxide to metallic iron,

3. Treating the reduced oxide product with a halogen acid, preferably hydrochloric acid, or otherwise suitably acidulating the reduced oxide product, as for example by th addition of ferrous chloride.

4. Heat-treating, at a temperature slightly higher than in the first heat-treatment, the acidulated product, preferably with a slight residual acid contact and a small amount of added carbonaceous material, in a reducing environment, less reducing in character than in the first heat-treatment and incapable of efiecting any further substantial change in the degree of reduction of the iron oxides present.

5. Annealing or slowly cooling the heat-treat ed product through the temperature range in which ferrous oxide is unstable and dissociates into metallic iron and magnetite.

1. PROPORTIONING THE INITIAL OIHDE MIXTURE Unlike the prior art sintered ferrites. which require rather precisely adjusted compositions of the various metal oxides if the desired magnetic and electrical properties are to be obtained, the proportioning of the various oxides in compositions of the invention is not critical and may be widely varied. This permits varying the proportions of the various metal oxides to attain the qualities desired for the conditions of use. However, since the primary object is to produce a composition of improved magnetic and electrical properties, ferric oxide is the essential component, and the other divalent metal oxides are merely modifying agents whose content depends primarily on the degree of change desired. In general, the modifying oxides may be classified in two groups:

Group I .Divalent metal oxides in the transition zone of the periodic system, and near or within the iron group, especially cobalt, nickel and manganese. Chromium falls within this class, but cannot easily be retained in divalent form. The atoms of these metals (with the possible exception of certain rare earths and the succeeding rare metal transition group) have electronic structures uniquely necessary for the appearance of ferro-magnetic quality. Thus, it is possible for combinations of these metals, including iron itself, to produce more strongly ferro-magnetic combinations than theindividual metals alone can attain. The oxides of these metals may therefore be defined as magnetic modifiers.

Group II.Divalent metal oxides capable of entering into the spinel or ferrite lattice, but of an electronic structure not essentially capable of ferro-magnetism. The most important examples are stable oxides of the second group of the periodic system, with especial emphasis on zinc and magnesium, but including divalent copper from the first sub-group. The oxides of these metals are essentially the modifiers of electrical conductivity in the ferrite lattice, and may therefore be defined as conductance modifiers. While divalent copper is primarily a conductance modifier, it may also be used as a ma netic modifier.

In the interest of simplicity, and further because fuller data is available with respect to them, nickel oxide (MO) is herein to be understood as representative of the magnetic modifiers, and zinc oxide (ZnO) as representative of the conductance modifiers.

The explanation herein given of the theory and principles underlying the method of the invention is supported and confirmed by extensive experimental research, but it is to be understood that this explanation is offered without prejudice and with no intention of restricting the invention thereto. It is now believed that the method of the invention takes advantage of the spontaneous decomposition of ferrous oxide at certain temperatures. Ferrous oxide is a phase in the iron-oxygen constitution diagram comprising the approximate composition FeO, usually, however, containing a somewhat greater oxygen content than corresponds to the ratio FezO, and hence may be regarded as a solution of ferric oxide (or magnetite) in ferrous oxide. It is stable only at temperatures above about 580 C., but may be quenched in the form existing above this temperature by cooling to below 300 C. In the quenched form, it it not ferro-magnetic but only somewhat para-magnetic. When the quenched form is annealed at a temperature between about 300 to 580 C., a reversion or dissociation of the ferrous oxide takes place, which may be represented by the equation:

in consequence of which the product of the reversion is strongly ferro-magnetic.

When the reversion of ferrous oxide to iron and magnetite takes place, there is no visible change in the material unless the anneal is greatly prolonged so that actual grain growth is possible. The reversion product is thus suitably designated crypto-crystalline, since evidence of the reversion can onl be obtained by magnetic measurements or special chemical tests. Since the reversion product contains metallic iron it may suitably be designated ferritic. The magnetite produced by the reversion is a ferrite of ferrous oxide, in the fundamental electronic and constitutional state of all ferromagnetic ferrites.

By the method of the invention, divalent metal oxides, capable of decreasing the magnetic hardness and electrical conductivity of ferrites, are dissolved in the ferrous oxide phase, and during reversion enter the ferrite lattice in such a way as to desirably modify the magnetic and electrical properties of the magnetite resulting from the reversion. Since the replacement of part of the ferrous oxide by other divalent metal oxides results in isomorphous mixtures or solutions without readily distinguishable solubility limits, there is no theoretical limit to the possible extent of such replacement. However, since only the ferrous oxide content of such a substituted mixture is capable of the desired reversion, a molar relationship of 3FeOzMO (where MO is the magnetic modifying oxide) is the lower limit of ferrous oxide that can result in a completely non-ferrous ferrite by reversion, as indicated in the equation:

SFeO-MO Fe MO-FezOi It has been found in practice that the molar relationship of 3FeOzMO represents approximately the point at which further substitution of a magnetic modifier for ferrous oxide causes progressive diminution of the magnetic quality of the composition. Nor is it necessary to go to lesser ratios of ferrous oxide to magnetic modifier than 3:1 for existing commercial specifications. Since the magnetic modifiers are much more expensive than ferric oxide, it is advantageous to limit the extent of substitution to that essential for obtaining the desired commercial specification. Accordingly, in practice, the preferred molar relationship of ferrous oxide to magnetic modifier is usually between 35:1 and 4:1.

The conductance modifiers, unlike the magnetic modifiers, are not necessarily all soluble in all proportions in the ferrous oxide phase. From the foregoing discussion of the adaptability of the magnetic modifiers to the ferrous oxide lattice, and its reversion, it is evident that any deficiency of the magnetic modifier below the ratio 3Fe0:MO can be replaced by a conductance modifier without impairing the homogeneity of the cryptic-crystalline ferrite, as for example in the molar relationship of 3Fe0: MO+ CO (where CO is the conductance modifying oxide). Nevertheless, it is possible, and in some instances desirable, to exceed the theoretical substitution limit of the conductance modified constituent in order to achieve a specified high quality factor designated as Q (hereinafter defined) even at the cost of some lessening of the permeability entailed by the resulting minor non-homogeneity. Economic considerations are less important for the conductance modifiers, since they are usually much cheaper than the magnetic modifiers. The commercial criterion is usually determined by the required Q value and loss specification.

With initial oxide mixtures of ferric oxide, zinc oxide and nickel oxide the actual weight percentage of ferric oxide may vary from 50 to 90%, although usually not exceeding about 70%. The ratio of zinc oxide to nickel oxide, in actual weight, may vary from about 2:1 to about 1:1. For example, very satisfactory compositions of the invention, for varied purposes, have been derived from initial oxide mixtures containing by actual weight (1) 58% F8203, 24% ZnO and 18% N10; (2) 70% F6203, 20% ZnO and 10% mo; (3) 77.4% FezOs, 14.9% ZnO and 7.7% N10; (4) 64% Fezoa, 24% ZnO and 12% NiO; 54% @3203, 33% ZnO and 13% N; and (6) 657 FerOa, 20% ZnO and 15% N10.

A carbonaceous material is preferably incorporated in the initial oxide mixture, and may suitably be dextrin, soluble starch, flour, cellulosic substances, etc. The amount of carbonaceous material present in the mixture should not be greater, under the predetermined conditions of prefiring, than necessary to reduce all of the ferric oxide to ferrous oxide, so that reduction to metallic iron is avoided. The actual quantity will depend on the rate of firing, furnace atmosphere, mode of packing in the furnace, and other variables, and is best determined empirically. Usually from 1 to 5% by weight of carbonaceous material, based on the total weight of the mixture, gives satisfactory results.

The formation of large discrete particles of metallic iron in the crystalline state (as distinguished from the crypto-crystalline state) is to be avoided in both heat-treatments, since they invariably lead to low Q values, probably because of the size and disposition of such crystalline iron particles, and because a conduction network of filamental form may appear in the composition if carried too far. A small amount of noncrypto-crystalline metallic iron produced in the pre-firing step is not too serious, since subsequent grinding, mixing and final firing in a controlled reducing atmosphere promotes uniformity in the final structure.

The initial oxide mixture is thoroughly mixed to produce a homogeneous dispersion of the three oxides, so that the subsequent reactions will take place uniformly throughout the entire mass of the mixture. Various types of mixing devices are suitablefor the purpose, among which may be .mentioned, by way of example, conventional dough-type mixers, Muller, Chilean and edgerunner type mills consisting essentially of a tical, say in 3 to 6 hours.

stone base, and ribbon type mixers consisting essentially of a series of blades or paddles that agitate a ribbon or moving bed of the mixture. In general, any conventional agitator or mixer which will produce a homogeneous mixture in which the three oxide constituents are uniformly dispersed may be employed. In lieu of dry mixing, the oxides may be mixed in the form of an aqueous slurry in conventional equipment therefor.

Commercial grades of the raw oxides are generally of satisfactory purity, and usually sufficiently fine in particle size for this stage of the method. However, if desired, the mixture, either during mixing or in a subsequent operation, may be ground to any desired degree of fineness. In general, a fineness of substantially all through to 200 mesh (standard Tyler screen) is satisfactory at this stage. The mixture is preferably agglomerated (preparatory to pre-flring) with the aid of a suitable binder, such as dextrin or equivalent carbonaceous binding agent. The inclusion of a carbonaceous material in the agglomerated mixture tends (during pre-firing) to assure greater uniformity of the reducing condition throughout the mass of the agglomerate, thus minimizing the possibility of greater reduction near the surface of the agglomerate than near the center.

2. FEE-FIRING The pre-flring or first heat-treatment step may be carried out in any suitable type of kiln or furnace. A kiln direct-fired with a combustible gas adapted to provide the contemplated temperature and reducing atmosphere may advantageously be used. Ordinary commercial city gas is satisfactory, and with such a gas a ratio of gas to air of from 3 to 4 volumes of air to one volume of gas provides a satisfactory reducing atmosphere. The following is a typical analysis of such a city gas which has been used (with the stated ratios of air) in practicing the pre-firing step of the method:

The pre-firing is conducted at a temperature between 800 C. and 1200 C., and usually more advantageously between 1050 C. and 1100 0., depending upon the characteristics it is desired to impart to the finished article, as explained hereinafter.

The duration of the pre-firing operation de pends to some extent upon the composition, the dimensions and the compactness or density of the agglomerates, and the amount undergoing heattreatment. Generally speaking, the agglomerates should be retained at the required tem perature for several hours, say 2 to 8 hours. Heating to that temperature may be carried out rapidly, say in twenty minutes or so, or gradually over a period of an hour or more. Cooling is effected as promptly as convenient and prac- The agglomerates mayadvantageously be placed in containers 15 inches long, 10 inches wide and 2 inches high. The specific heats of the oxide mixtures used in the method of the invention are such that the heavy grinding wheel running over a metal or 7 agglomerates can be brought to a uniform temperature of 800-1200 C. in 20 minutes, if desired. With such agglomerates, the entire pre-firing cycle is from 6 to 14 hours, to 2 hours for bringing the agglomerates to the required prefiring temperature, a 2 to 6 hour retention period at that temperature, and 3 to 6 hours for cooling the heattreated agglomerates to room temperature.

If the products of the pre-firlng and final-firing steps are quenched very rapidly, they are nonmagnetic, or only slightly so. This demonstrates that the ferro-magnetic components of the composition of the invention are not formed directly in either firing step, in contrast to the heretofore customary methods of producing sintered ferrites. Consequently, in the method of the invention, it is unnecessary to use an extremely high temperature in the pre-firing step, or an excessively prolonged retention period at the pre-firing temperature for satisfactory homogenizing.

The ferrous oxide phase produced by the prefiring is much more fusible than crystalline ferrites. and hence the charge is in a sense in a semifiuid state, and rapid inter-diffusion of the constituents takes place, as contrasted with the very slow attainment of homogeneity in the present practices for producing ferrites. Too high temperatures may cause actual liquation of all or part of the charge, which is undesirable. The temperature at which homogeneity is rapidly reached without collapse of the charge is usually between 850 and 1100 C. and about 1050 C. is the normal operating temperature for most mixtures.

The magnetic modifying oxides appear to have little effect on the homogenizing time or on the softness of the charge during pre-firing. While they may produce a slight increase of fluid quality, the effect is not sufiicient to require any change of firing procedure as the amount of the magnetic modifier is altered through the specifled range.

On the other hand, the electrical modifying oxides have relatively high melting temperatures (with the exception of cupric oxide CuO, which tends to decompose), and they hence tend to increase the viscosity of the ferrous oxide phase, at least in the usual composition ranges. Thus, somewhat higher pre-firing temperatures are required when increased amounts of the electrical modifier are included in the initial oxide mixture.

The major part of the ferrous oxide formation seems to take place during the time used to attain the ultimate firing temperature, and to be substantially completed during retention at that temperature. Preferably, the residual carbon of the carbonaceous binder is mainly relied upon for the reduction of the ferric oxide to ferrous oxide, since too great reliance on the reducing atmosphere tends to produce a reduced product of inferior uniformity. It is not necessary that all of the ferric oxide be converted to ferrous oxide, since in fact the ferrous oxide phase always contains some higher oxide of iron (above the FeO ratio). At a pre-firing temperature of about 1050 C., the oxygen content of the ferrous oxide phase is between 23.1 and 24.5%. The oxygen content of ferrous oxide (FeO) is 22.27%, and of ferric oxide (Fezos) is 30.06%. The oxygen de- 8 duced product of the pro-firing step is subsequently refired, the cooling rate is not critical, and may be conducted as rapidly as the furnace structure and other considerations permit.

2A. GRINDING THE REDUCED PRODUCT The reduced product is ground to a predetermined particle size in order to insure better uniformity of treatment in the subsequent operations, as well as to provide such particle or crystal size as is best suited to the particular field of use of the finished composition. Generally speaking, in the use of the composition as a magnetic core, the higher the frequency of the energizing current, the finer should be the grinding. For example, at lower frequencies up to kilocycles, grinding to a fineness of through (i. e. less than) 100 mesh is usually satisfactory. With frequencies of the order of 1000 kilocycles, grinding should be carried to a fineness of at least through 200 mesh. With frequencies of the order of 100 megacycles, grinding may advantageously be carried to an average particle size of around 1 micron, or even less.

while fine grinding to the ultimate desired particle size of the finished composition is preferably carried out at this stage, i. e. following the prefiring step, it may be carried out after the acid treatment. However, grinding prior to the acid treatment is generally more efiective. Even if it is desired to grind the composition after the acid treatment, some preliminary grinding, say to through 100 mesh, should precede the acid treatment.

3. ACID TREATMENT The ground reduced product is next subjected to treatment with a halogen acid. Commercial muriatic (hydrochloric) acid having a specific gravity of 1.124 (16 B.; 24.57% HCl) is suitable for the purpose. The ground reduced product is introduced into a suitable container, and the acid is poured over it. About 2.5 gallons (22.5 pounds) of acid is used for each 50 pounds of ground reduced product. The ground reduced product is allowed to remain in contact with the acid for a period of about one hour. However, longer treatment periods have no noticeable deleterious effects, nor are they beneficial.

At the end of the acid treatment, the supernatent acid is decanted, and is replaced by wash water, which, in turn, is decanted, and replaced by further wash water. This washing treatment is continued. until the pH of the decanted wash water has reached a value of about 3. In practice. this takes about six washings with fresh water, in the manner described. It is important that washing be carried out until the residual liquor associated with the product has a pH of about 3. If too much acid remains associated with the product, drying at reasonable temperatures is difiicult. On the other hand, it is desirable to retain a certain amount of the acid liquor in the acidulated product.

For optimum results, the acid treatment requires concentrated hydrochloric acid, e. g. 16 B. commercial muriatic acid. The electrical properties of the final compositions are distinctly inferior when the acid treatment is carried out with dilute hydrochloric acid. When the acidtreating solution contains 50% and more of the 16 B. hydrochloric acid, it is impossible to dry the acidulated product unless the product is washed as hereinbefore described. Therefore, while it is possible to use acid-treating solutions containing in excess of 50% (but less than 100%) 16 B. hydrochloric acid, it appears that the preferable practice is to use the concentrated acid with subsequent washing until the residual liquor remaining associated with the product has a pH of about 3. v

The acid treatment promotes rapid sintering and homogenizing during the final firing step.

The composition of the acidulated product differs only minutely from the untreated reduced prodnot, and the utility of the acid treatment is believed to be (a) conversion of the surfaces of the solid particles to an amorphous or active form and/or (b) the promotion of grain growth by the presence of small quantities of volatile metallic salts as contaminants on the surface of the particles. Insufiicient washing of the acidulated product may leave therein too great a quantity of contaminants, and cause blistering during final firing. When the residual wash water associated with the product reaches a pH of 3, the product sinters rapidly and effectively without blistering.

3A. PREPARATION FOR FINAL FIRING The acidulated product is air dried by any conventional means preparatory to subsequent treatment. Preferably, a suitable binder is mixed with the product to facilitate pressing or forming and to form a sufiiciently self-sustaining mass for the final firing step. While various binders (preferably carbonaceous in character) are suitable, phenol formaldehyde resins have been found particularly advantageous. About 3% by weight of the dry resin is usually satisfactory, although as little as 1% may be used. While the dry resin may be directly added to and mixed withthe acidulated product, it is preferred to add the resin in the form of a solution in a suitable solvent, such as acetone or any other appropriate solvent for the resin. A minimum amount of solvent should be used, the aim being to effect uniform dispersion of the resin and so wet (but not slurry) the particles that they tend to agglomerate and stick together and remain self-sustaining during subsequent treatment. When the resin solution has been uniformly dispersed throughout the mass of the acidulated product, the resin solvent is removed by a simple drying operation. The inclusion of some carbonaceous material in the acidulated product contributes beneficially to the final firing step, since it insures a proper reducing environment throughout the mass of the product. On the other hand, only-such amount of carbonaceous material should be included as will be burned out under the conditions of the final firing step, since residual carbonaceous material adversely affects the properties of the finished composition.

The dried acidulated product (with added binder) is pressed by any suitable apparatus into the shape and size of the final article (e. g. magnetic core). Due allowance should be made for shrinkage during final firing, which may range from a few per cent up to 25%, depending upon the composition itself, conditions of prefiring etc. For example, where shrinkage is known to be 10%, the size of the pressed article will be 110% of the desired size of the finished article. The surface of the pressed article may be coated with a ceramic-like material in order to provide a highly resistive surface and thereby increase the resistance of the final article. The coating may be conveniently efiected by dipping the article in an aqueous suspension of talc and feldspar, in the proportion of about parts by 10 weight of talc and parts of feldspar. In those cases where high surface resistance of the final article is not a factor to be taken into consideration, the surface coating may be omitted.

4. FINAL FIRING The final firing or heat-treatment step is similar in character to the pre-firing step except that it is carried out at a somewhat higher temperature and in a slightly less reducing environment. The final firing may conveniently be conducted in a kiln or furnace similar in design to that employed in the pre-firing step and utilizing a nearly neutral or mild reducing atmosphere slightly less reducing in character than that prevailing in the pre-firing step. If the reducing environment of the final firing is too strong (i. e. too reducing in character) excessive shrinkage of the article and the formation of pores takes place with a resultant impairment in permeability. Too weak a reducing environment allows oxidation to take place,

causing swelling of the article and impairment of its desired characteristics. As in the pre-firing step, any type of combustible gas that can be burned with air to provide the contemplated temperature and nearly neutral or mild reducing atmosphere is satisfactory for final firing, the aforementioned city gas being quite satisfactory for the purpose. Generally, in both firing steps, the gas and air are pre-mixed before being introduced into the kiln. Where the pre-firing step is carried out with an air to gas volume ratio of 3 to 1, the final firing step may be carried out with an air to gas volume ratio of 3.5 to 1, and where the air to gas volume ratio of the pre-firing step is 4 to 1, the air to gas volume ratio of th final firing step may be about 4.25 to 1. The temperature of the final firing step is always somewhat higher than in the pre-firing step, and generally lies in the range of from about 1100" C. to about 1450 C. The temperatures of the two firing steps should be correlated in order to obtain the best results, and generally the same considerations cover selection of the final firing temperature as in the selection of the pre-firing temperature, as hereinafter more fully explained. Generally speaking, pre-firing homogenizes the constituents of the charge, and final firing homogenizes the grains as a structural entity. In order to minimize the demagnetizing effect of internal poles, physical discontinuities within the structure must be eliminated to the greatest possible extent. It is possible to compensate somewhat for lack of homogeneity in too rapid pre-firing by increasing the time or temperature, or both, of final firing, and where production conditions necessitate such compromise, final firing must be adjusted to the conditions of pre-firing for the particular batch of articles under processing. This is not, however, the best procedure. The criterion that must be used is naturally the properties of the final article, both as to permeability and Q value. The minimum temperature at which complete homogeneity can be attained in final firing is in the vicinity of 1100 0., corresponding to about the maximum temperature of pre-firing. Higher temperatures shorten the required time of final firing, but the temperature at which the article softens and tends to alter its shape must not be too closely approached. The minimum melting temperature in the iron-oxygen system is about 1380 C., and the minimum temperature at which ferrous oxide and magnetite may be at equilibrium without melting is about 1430 C. The presence of the modifying oxides, and especially of the dimcultly fusible electrical modifiers, tends to raise the softening temperature somewhat. In general, the maximum safe operating temperature of final firing is about 1400 0., although with some special compositions the temperature may be as high as 1450 C.

The articles undergoing final firing preferably and usually contain a small amount of carbonaceous material, as hereinbefore mentioned, and the furnace atmosphere may then be less reducing. The conditions of final firing are such that carbonaceous residues are eliminated in gaseous form without exercising any reducing action upon the metal oxides, so that the oxygen deficiency resulting from pre-firing is maintained. As in pre-firing, direct reduction of any substantial quantity of the iron oxides to large discrete particles of metallic iron is to be avoided as deleterious to the Q value of the finished article. A typical final firing cycle comprises heating to the ultimate temperature in four hours, retention at that temperature for eight hours, and cooling through a period of four hours. The elimination of residual carbon from the carbonaceous binder is generally, and preferably, completed before the ultimate temperature retention period is attained.

Considering the sluggishness of the reversion of unmodified ferrous oxide, annealing of the articles after final firing would be expected to be a critical matter. It has been found, however, that prolonged annealing is unnecessary, and when cooling is conducted over a four hour period, the reversion to magnetic form is accomplished. The modified ferrous oxide has a less distinct reversion temperature than unmodified ferrous oxide. For example, when articles of the invention are removed from the final firing furnace at various temperatures during the cooling period, and quenched, no sharp reversionary temperature, comparable to the 580 C. temperature for ferrous oxide reversion, is found, but instead the articles taken out at progressively lower temperatures display a rather difiuse and gradual increase of term-magnetism. Thus, it appears that in the method of the invention, the reversion of the ferrous oxide starts shortly below the final firing temperature and is completed at about 300 C. The phenomenon depends on the degree of modification of the ferrous oxide, the smaller the proportions of the modifying oxides in the initial oxide mixture, the closer the approach to unmodified ferrous oxide behavior with a sharp reversion temperature range. The degree of oxygen deficiency produced during prefiring is also a factor in the phenomenon. Furthermore, it appears that the modifying oxides accelerate the rate of reversion, so that no longer than a four hour cooling period is required for substantially complete conversion to magnetic form.

The heat-treatments of the initial oxide mixture and final shaped articles may be carried out in apparatus other than kilns direct-fired with combustible gas. Any apparatus that will give the required elevated temperatures and the contemplated atmospheres is satisfactory. For example, the required temperatures may be obtained with an electrically heated furnace, oven or kiln, and the contemplated atmospheres may be obtained by a mixture of hydrogen or carbon monoxide (or both) with nitrogen in suitable proportions.

Compositions of the invention resemble cores made of powdered iron to the extent that they i2 are ferritic, that is contain metallic iron, but differ from such cores in that (1) they contain no non-ferro-magnetic bonding agent and attendant physical non-homogeneities, with consequent loss of permeability due to internal poles and multiple air gaps, and (2) the iron content is introduced internally by spontaneous decomspects 1. The various components are not restricted to specific proportions in order to attain a high quality of ferro-magnetism.

2. The magnetic structure is not obtained directly by crystallization to a specified ferrite at high temperature, but results from an internal change taking place during annealing or cooling,

and at a lower temperature than that of the heat-treatment, as a consequence of which a crypto-crystalline homogeneous structure is obtained, rather than the gross mono-crystalline structure characteristic of the sintered ferrites and resulting from slow growth of micro-crystalline material.

3. Because of its crypto-crystalline structure, the compositions of the invention are physically homogeneous, but atomically heterogeneous.

'4. Because of the possible variation in composition and the inherent ferritic quality, compositions of the invention are adjustable to commercial design variations over a wide range of magnetic and electrical properties for use throughout a. wide range of circuit frequencies.

5. Because of its ferritic quality and possibility of control. of Curie point over considerable ranges of temperature, cores and other articles having desirable temperature coefficients of permeability change can be readily produced for use in heavy duty or power circuits.

6. The magnetic permeability of cores and other articles made of the composition of the invention remains substantially constant, for all practical purposes, over the frequency range extending approximately from commercial power frequencies (e. g. 60 cycles) to the very high frequencies of several megacycles (e. g. 5 megacycles).

The magnetic compositions of the invention are principally useful in magnetic cores, such as transformer or radio frequency cores, as well as in many other products finding an application in electronic and radio work. The compositions also find application in generator and motor parts such as pole pieces and armatures.

Generally, the compositions find application in i n) may be defined as the ratio of the self-inductance of a coil having a magnetic core of given size and shape designed therefor to the self-inductance of the same coil with an air core. It is therefore a permeability averaged over the entire magnetic cycle. The quality factor Q may be defined as in which L is the self-inductance of the coil in air, Lo is the self-inductance of the coil with its intended magnetic core, F is the frequency of the exciting magnetic field and R is the equivalent series resistance of the coil, including the copper resistance of the coil itself (increased by skin effeet) and a component due to dielectric losses in the core, in the insulation of the winding, etc.

It is apparent that the quality factor Q and effe'ctive permeability as herein defined are not constants for the material itself of which the core is made, but are dependent upon the size and shape of the core in which they are measured and upon the coil into which the core is inserted for the purpose of measurement. Whenever hereinafter comparisons of permeabilities and of Q values are made, the data was obtained under conditions making these comparisons representative of inherent qualities of the core materials under consideration.

Cores made of the compositions of the invention possess desirably high values of permeability and Q, as the examples presently to be described will show. An important consequence of these superior electrical characteristics is the fact that cores of the compositions of the invention can for a given desired performance he made much smaller than cores of prior art materials. Thus, by means of the invention, a powdered iron core one inch long and inch in diameter can be replaced by a core less than 0.2 inch in diameter and 0.5 inch long. In additiomthe weight of such a core is only about 60% of the weight of a corresponding powdered iron core.

The invention is of particular advantage in the so-called long cores, that is in cores used for frequency tuning, such as cores of coils used to tune the local oscillators of broadcast receivers over the broadcast band. The powdered iron cores of the prior art are barely able to attain this range of frequency change, and are generally of the order of 0.180 inch in diameter by 1.5 inches in length, and are made of the most expensive material for such a range. This necessitates coil forms with a wall thickness of the order of 0.008 to 0.010 inch (carefully ground in many cases), and in which the winding must be most carefully done to maintain the wire very close to the coil form. The clearance between the coil form and the core must be the minimum possible. just enough to prevent binding. A long core made of a composition of the invention is capable of tuning a circuit in which its coil is connected from considerably below the 455 kilocycle intermediate frequency of standard broadcast receivers to beyond the 2200 kilocycle band. With cores of the invention, thick walled coil forms maybe used, without special winding being required, the core being 1 inch long and 0.25 inch in diameter. As a further example, a core of the invention permits the construction of an intermediate frequency transformer unit having an overall size of approximately 0.375 in. by 0.375 in. by 1 in., as contrasted with the standard indesign of approximately 1 in. by 1 in. by 2.5 inches.

A typical composition of the invention-where the initial oxide mixture was 58% FezOa, 24% ZnO and 18% Ni0had (when tested in a field of 1400 kc.) an eil'ective permeability of 7.0, and a Q value of 305. This corresponds to a true permeability in excess of 400. A composition similarly processed from an initial oxide mixture of 94% R202 (in excess of the inventions range), 3% ZnO and 3% N10 had an effective permeability of 3.05 and a Q value of 89, when similarly tested. A composition similarly processed from an initial oxide mixture of 25% FezOa (below the inventions range), 41 ZnO and 34% NiO had an effective permeability of 1.05 and a Q value of 104.

Cores processed from an initial oxide mixture of 70% F6203, 20% ZnO and 10% N10 have a high load characteristic. In other words, a considerably greater flux density is required to cause the core to saturate. Specifically, the load characteristic of a 0.75 inch core exceeds by 2 to 2.5 that of a corresponding prior art core.

A core processed from an initial o-xide mixture of 57% Fez-0:, 38% ZnO and 5% N10 had an effective permeability of 1.02 and a Q value of 97, illustrating the unfavorable effect of a zinc oxide to nickel oxide ratio greater than 2 to 1.

It has been found that the permeability and Q value of compositions processed from initial oxide mixtures with a zinc oxide to nickel oxide ratio greater than 2 to 1 vary greatly with temperature, both Q value and permeability decreasing objectionably as the temperature increases. For example, a core processed from an initial oxide mixture of 54% F6203, 33% ZnO and 13% N10 had an effective permeability of 7.68 and a Q value of 206. However, at 212 F. the Q value dropped to less than V of its former value, and the effective permeability was only about 1. Such large variations in Q value and effective permeability render the core unusable in applications subject to large temperature changes. With a zinc oxide to nickel oxide ratio of 2 to 1 in the initial oxide mixture, the Q value of the processed composition remains constant up to a temperature of about 350 F. and varies negligibly between 350 and 480 F., while permeability varies by only about 10% up to 350 F., and then drops rapidly from 400 to 480 F. With zinc oxide to' nickel oxide ratios between 2 to 1 and 1 to 1, Q values increase with temperature increase, while permeability remains relatively stable. Thus, as the ratio of zinc oxide to nickel oxide is decreased from 2 to 1 towards 1 to 1, the Q value tends to rise with an increase in temperature. In many instances this is a desirable factor as it overcomes some of the other losses that are inherent in electrical circuits due to temperature rise. On the other hand, the permeability of such processed compositions tends to remain constant with temperature increase. The eiIects are more pronounced as the ferric oxide content of the initial oxide mixture is increased from about 57% towards its upper limit. Very satisfactory compositions of the invention with these properties have been processed from initial oxide mixtures containing from 70-82% F6203 with zinc oxide to nickel oxide ratios between 2 to 1 and 1 to 1. With zinc oxide to nickel oxide ratios less than 1 to 1, the Q value increase as the temperature increases is more pronounced.

The following examples illustrate the relatively termediate frequency unit of thebest prior art 7 great constancy of permeability and Q value over a wide temperature range that can be obtained with certain compositions of the invention:

A core processed from an initial oxide mixture of 77.4% F8503, 14.9% ZnO and 7.7% NiO had, at room temperature, an effective permeability of 23.5 and a Q value of 90. At a temperature of 230 F. the effective permeability was 24.3 and the Q value was 104. Another core processed from an initial oxide mixture of 70% F8203, 20% ZnO and NiO had, at room temperature, an effective permeability of 10.2 and a Q value of 65. At 176 F., the effective permeability was 11.6 and the Q value was 65. At 248 F., the effective permeability was 10.5 and the Q value was 51.

For special applications (essentially low frequency), additional metallic iron may advantageously be included in the composition. As previously intimated, reduction during pre-firing may, if desired, be carried to the extent of reducing some ferrous iron to metallic iron. However, it is generally preferable to add the metallic iron in the form of powdered iron. This may advantageously be done by adding powdered iron up to as much as 25% by weight to the reduced oxide product. This is possible and effective in the method of the invention because the controlled reducing atmosphere of final firing inhibits any distortion in the finished article.

In lieu of the acid treatment hereinbefore described, from 0.1% up to several percent by weight of anhydrous ferrous chloride may be added to the ground reduced product. Alternatively, acidulation of the ground reduced product may be omitted, in which case that product is directly subjected to final firing. However, the hereinbefore enumerated advantages of the acid treatment are such that it is generally desirable to carry it out, except in special cases where economic or other considerations offset the advantages of the acidulating treatment.

The temperature conditions of the pre-firing step, in processing the oxide mixtures in accordance with the invention, are very important and greatly affect the electrical properties of the final composition. While a pre-firing temperature of 800 to 1200 C. is generally satisfactory, for certain specific purposes, the optimum pro-firing temperature should be from about 1050 C. to about 1100" C. For example, in producing transformer cores for use in relatively low frequency ranges, that is between '1 and kilocycles, the final composition should possess high permeability, only a fairly high Q value, and a relatively constant value for the permeability and Q value over a relatively wide range of temperature, and in the production of compositions for such purpose the optimum pre-firing temperature is 1100 C. While this temperature can be varied by about 50 C. in either direction, it should not be varied more in order to produce the best results. High frequency radio cores, on the other hand, are not subjected to extreme variations in temperature, and it is therefore not as important that the composition used for such cores have the consistency of electrical properties over such a widely varying temperature range as is required for transformer cores. Radio frequency cores must, however, possess a relatively high Q value and a high value for permeability. In processing compositions for such purpose, the optimum prefiring temperature is 1050 0., with a permissible variation of 50 C. in either direction, but preferably not more, for optimum results.

The following tables illustrate the effect of the temperatures of pre-firing and final firing Table I A.. I Temp. of Temp. of Inductance Pro-Fire Final Fire (111 1111111- I C.) O.) henries) i I 900 1, 240 120 1, 000 1, 240 210 1, 1, 240 180 900 l, 290 165 1,000 1, 290 415 1,100 1,290 340 t l 900 1,340 240 f 1, 000 1, 340 560 I 1,100 1,340 290 i 900 1,390 320 1 1,000 1,390 125 1 1,100 1,390 385 i 1, 000 1, 450 586 l 1 As a basis for direct measurement th e inductance figure 01340 corresponds to a true permeability of 452.

Table II 5 Temp. of Temp. of

' Pre-Firc Final Fire Q As hereinbefore mentioned, grinding to the contemplated final particle size is preferably carried out after pre-firing, and prior to the acid treatment, although the acidulated product may, if desired, be ground to the final particle size. Where it is desired to grind the acidulated product, the reduced product of the pre-firing step should be ground to a fineness of through 100 mesh prior to the acid treatment.

The relationship of grinding to final firing is important. A very fine grind, coupled with a relatively low final firing temperature produce compositions possessing the highest Q values at the highest frequencies. This relatively low final firing temperature should be closely maintained so that the very fine particle or crystal size is maintained.

As hereinbefore stated, there is no final firing temperature at which all electrical properties of the final composition will be at an optimum, and hence if superior temperature drift characteristics are desirable for a particular purpose, certain other properties, such for example as high Q value, mustbe sacrificed to some extent. Thus, in manufacturing a composition for use in transformer cores where the pro-firing temperature was about 1100 C., a final firing temperature of 7 about 1300 C. gives the best results for optimum temperature drift characteristics and inductance which are of primary importance in transformer cores. On the other hand, in manufacturing a composition for use in high frequency radio cores, where the temperature drift characteristic is not of primary importance, but Q value and permeability must be high, optimum results are obtained with a pre-firing temperature of 1050 C. and a final lower.

1 claim:

1. As a new article of commerce, a magnetic composition possessing useful magnetic and electrical properties and composed principally of ferro-magnetic material containing a mixed ferrite and metallic iron in crypto-crystalline form while substantially free of large discrete particles of metallic iron. 2. As a new article of commerce, a magnetic composition possessing useful magnetic and electrical properties and composed principally of a homogeneously dispersed mixture of metallic iron in orypto-crystalline form and a mixed ferrite in which zinc oxide and nickel oxide have replaced at least part of the ferrous oxidein the ferrite lattice.

3. As a new article of commerce, a magnetic composition possessing useful magnetic and electrical properties and composed principally of (1) a mixed ferrite in which divalent metal oxide modifiers of magnetic and electrical properties have replaced at least part of the ferrous oxide in the ferrite lattice and (2) metallic iron in crypto-crystalline form while substantially free of large discrete particles of metallic iron.

4. A magnetic article comprising a compacted magnetic composition composed principally of crypto-crystalline ferritic ferrite in which the ferritic component is metallic iron in a finer state of comminution than in powdered iron and the ferrite component contains divalent metal oxide modifiers of magnetic and electrical properties.

5. A magnetic article comprising a compacted magnetic material composed principally of a homogeneously dispersed mixture of metallic iron in crypto-crystalline form and a mixed ferrite containing zinc oxide and nickel oxide.

6. A magnetic article comprising a compaced magnetic composition composed principally of ferro-magnetic material containing a mixed ferl7 firing temperature of 1250 C. or

cooling the heat-treated product in the course of which spontaneous dissociation of at least part of the ferrous oxide takes place to form cryptocrystalline iron. I

9. The method of claim 8 in which the first heat-treatment is carried out at a temperature of 1050 to 1100 C. and the second heat-treatment is carried out at a temperature of 1200 to 1300 C.

10. The method of claim 8 inwhich the electrical modifying metal oxide and the magnetic modifying metal oxide are present in the initial oxide mixture in the relative proportions by weight of between about 2 to l and about 1 to 1, respectively.

11. The method of claim 8 in which the electrical modifying metal oxide is zinc oxide and the magnetic modifying metal oxide is nickel oxide.

12. The method of claim 11 in which the relative proportions by weight of zinc oxide and nickel oxide are between about 2 to l and about 1 to 1, respectively.

13. The method of preparing a magnetic composition which comprises intimately mixing ferric oxide, zinc oxide and nickel oxide in the proportions by weight of from 50 to 90% of ferric oxide and a weight ratio of zinc oxide to nickel oxide of between about 2 to 1 and about 1 to l, subjecting the .oxide mixture to heat-treatment at a temperature of 800 to 1200 C. in a reducing environment sufiicient only to effect reduction of a large part of the ferric oxide to ferrous oxide,

' treating the reduced product with an acidulating rite and metallic iron in crypto-crystalline form while substantially free of large discrete particles of metallic iron and possessing a substantially constant magnetic permeability over a frequency range from 60 cycles to several megacycles.

7. A magnetic article comprising a compacted mixture of (1) powdered metallic iron and (2) a magnetic composition composed principally of ferro-magnetic material containing a mixed ferrite and metallic iron in crypto-crystalline form while substantially free of large discrete particles of metallic iron, the proportion of powdered iron in the mixture being up t 25% by weight, said article possessing a substantially constant magnetic permeability over a wide range of frequencies.

8. The method of preparing a magnetic composition which comprises intimately mixing ferric oxide, an electrical modifying metal oxide and a magnetic modifying metal oxide to produce an intial oxide mixture in which the proportion of iron oxide is from 50 to 90% by weight, sub jecting the oxide mixture to heat-treatment at a temperature of 800 to 1200 C. in a reducing environment sufilcient only to effect reduction of a large part of the ferric oxide to ferrous oxide, grinding the reduced product to a fineness of substantially all through 100 mesh, subjecting the ground reduced product to a second heat-treatment at a temperature of 1100 to 1450 C. in a reducing environment less reducing in character than in th first heat-treatment, and gradually agent, subjecting the acidulated product to heattreatment at a temperature of 1100 to 1450 C. in a reducing environment less reducing in character than in the first heat-treatment, and gradually cooling the heat-treated product in the course of which spontaneous dissociation of at least part of the ferrous oxide takes place to form cryptocrystalline iron.

14. The method of preparing magnetic articles which comprises intimately mixing ferric oxide. an electrical modifying metal oxide and a magnetic modifying metal oxide to produce an initial oxide mixture containing from 50 to by weight of ferric oxide and in which the weight ratio of the electrical and magnetic modifying metal oxides is between about 2 to l and about 1 to l, subjecting the oxide mixture to heat-treatment at a temperature of 800 to 1200 C. in a reducing environment sufilcient only to effect reduction of a large part of the ferric oxide to ferrous oxide. grinding the reduced product to a fineness of at least substantially all through mesh, treating the ground reduced product with an aqueous solution of hydrochloric acid, compacting and forming the acidulated product with some residual acid content and of a fineness substantially all through 100 mesh into magnetic articles, subjecting the magnetic articles to heattreatment at a temperature of 1100 to 1450 C. in a reducing environment less reducing in character than in the first heat-treatment, and gradually cooling the heat-treated articles in the course of which spontaneous dissociation of at least part of the ferrous oxide takes place to form crypto-crystalline iron.

15. The method of claim 14 in which the acid treatment is carried out with 16 B. hydrochloric acid and the acidulated product is washed until the liquid associated therewith has a pH of about 3.

16. The method of claim 14 in which a carproduct prior to compacting and forming.

17. The method of preparing a magnetic composition which comprises intimately mixing ferric oxide, an electrical modifying metal oxide and a magnetic modifying metal oxide to produce an initial oxide mixture in which the proportion of ferric oxide is from 50 to 90% by weight, subjecting the oxide mixture to heat-treatment at a temperature of 800 to 1200 C. in a reducing environment with a heat-treatment cycle of 6 to 14 hours sufiicient only to effect reduction of a large part of the ferric oxide to ferrous oxide, treating the reduced product with an acidulating agent, and subjecting the acidulated product to heat-treatment at a temperature of 1100 to 1450 C. in a reducing environment less reducing in character than in the first heat-treatment with a heat-treatment cycle of 12 to 18 hours during about the last third of which the charge is gradually cooled from the elevated temperature to room temperature in the course of which spontaneous dissociation of at least part of the ferrous oxide takes place to form crypto-crystalline iron.

18. The method of preparing magnetic articles which comprises intimately mixing ferric oxide, zinc oxide and nickel oxide to produce an initial oxide mixture containing from 50 to 90% ferric oxide and in which the weight ratio of the zinc and nickel oxides is between about 2 to 1 and about 1 to 1, subjecting the oxide mixture to heat-treatment at a temperature of 800 to 1200 C. in a reducing environment sufiicient only to effect reduction of a large part of the ferric oxide to ferrous oxide, grinding the reduced product to a fineness of at least substantially all through 100 mesh, treating the ground reduced product with an aqueous solution of hydrochloric acid, compacting and forming the acidulated product with some residual acid content and of a fineness substantially all through 200 mesh into magnetic articles, subjecting the magnetic articles to heat-treatment at a temperature of 1100 to 1450 C. in a reducing enviromnent less reducing in character than in the first heat-treatment, and gradually cooling the heat-treated articles in the course of which spontaneous dissociation of at least part of the ferrous oxide takes place to form crypto-crystalline iron.

19. The method of preparing magnetic articles which comprises intimately mixing ferric oxide, zinc oxide and nickel oxide to produce an initial oxide mixture containing from 50 to 90% ferric oxide and in which the weight ratio of the zinc and nickel oxides is between about 2 to 1 and about 1 to 1, subjecting the oxide mixture to heattreatment at a temperature of 800-to 1200 C. in a reducing environment suflicient only to effect reduction of a large part of the ferric oxide to ferrous oxide, grinding the reduced product to a. fineness of at least substantially all through 100 mesh, treating the ground reduced product with concentrated hydrochloric acid, washing the acidulated product until the liquid associated therewith has a pH of about 3, drying the washed acidulated product, compacting and forming the dried acidulated product into magnetic articles, subjecting the magnetic articles to heat-treatment at a temperature of 1100 to 1450 C. in a reducing environment less reducing in character than in the first heat-treatment, and gradually cooling the heat-treated articles in the course of which spontaneous dissociation of at least part of the ferrous oxide takes place to form cryptocrystalline iron.

20. The method of preparing a magnetic comric oxide, zinc oxide and nickel oxide in the proportions by weight of to 90% ferric oxide and a weight ratio of zinc oxide to nickel oxide of from 2:1 to 1:1, subjecting the oxide mixture to heat-treatment at a temperature of 800 to 1200 C. in a reducing environment sufiicient only to effect reduction of a large part of the ferric oxide to ferrous oxide and until the oxygen loss based on the ferric oxide of the mixture is from 1 to 7% by weight, grinding the reduced product to a fineness of substantially all through 100 mesh, treating the ground reduced product with an acidulating agent, subjecting the acidulated product to heat-treatment at a temperature of 1100 to 1450 C. in a reducing environment less reducing in character than in the first heattreatment step, and gradually cooling the heattreated product in the course of which spontaneous dissociation of at least part of the ferrous oxide takes place to form crypto-crystalline iron.

21. The method of preparing magnetic articles which comprises intimately mixing ferric oxide. zinc oxide and nickel oxide in the proportions by weight of 50 to 90% ferric oxide and a weight ratio of zinc oxide to nickel oxide of from 2:1 to 1:1, subjecting the oxide mixture to heattreatment at a temperature of 800 to 1200 C. in a reducing environment sufficient only to effect reduction of a large part of the ferric oxide to ferrous oxide, grinding the reduced product to a fineness of at least substantially all through 100 mesh, treating the ground reduced product with concentrated hydrochloric acid, washing the acidulated product until the residual liquid associated therewith has a pH of about 3, drying the washed acidulated product, mixing a carbona ceous binder with the dried acidulated product, compacting and forming the resulting mixture into magnetic articles, subjecting the magnetic articles to heat-treatment at a temperature of 1100 to 1450 C. in a reducing environment less reducing in character than in the first heattreatment step, and gradually cooling the heattreated articles in the course which spontaneous dissociation of at least part of the ferrous oxide takes place to form crypto-crystalline iron.

22. The method of preparing magnetic articles which comprises intimately mixing ferric oxide and at least two divalent metal oxides capable of modifyin the magnetic and electrical properties of ferrous ferrite by replacement of ferrous oxide and incorporationg a small amount of carbonaceous material in the oxide mixture, subjecting the oxide mixture to heat treatment at a temperature of 800 to 1200 C. in a reducing environment capable of reducing a large part of the ferric oxide to ferrous oxide, grinding the reduced product to a fineness of at least substantially all through mesh, compacting and forming the ground reduced product and a small amount of added carbonaceous material into magnetic articles, subjecting the magnetic articles to heat-treatment at a temperature of 1100 to 1450 C. in a reducing environment less reducing in character than in the first heat-treatment, and gradually cooling the heat-treated articles in the course of which spontaneous dissociation of at least part of the ferrous oxide takes place to form crypto-crystalline iron.

23. The method of preparing a magnetic composition which comprises intimately mixing ferric oxide, an electrical modifying metal oxide and a magnetic modifying metal oxide to produce an initial oxide mixture in which the proportion of iron oxide is from 50 to 90% by weight, efiecting reduction of a substantial portion of the ferric oxide content of the oxide mixture to ferrous oxide, and subsequently eifecting thermally induced spontaneous dissociation of at least part of said ferrous oxide in the oxide mixture to form therein a substantially homogeneous dispersion of metallic iron in, crypto-crystalline iron.

24. The method of preparing a magnetic composition which comprises intimately mixing ferric oxide, an electrical modifying metal oxide, a magnetic modifying metal oxide and carbonaceous material to produce an initial oxide mixture in which the proportion of iron oxide is from 50 to 90% by weight and in which the proportion of carbonaceous material is from 1 to 5% of the total weight of the mixture but not greater than necessary to reduce all of the ferric oxide to ferrous oxide, pre-firing said mixture at a temperature between 800 C. and 1200 C., intimately admixing with the pre-fired mixture at least 1% by weight of a carbonaceous binder m9.- terial but not more than will be burned out under the subsequent firing step, compacting the resulting mixture, and subjecting the compacted mixture to firing at a. temperature higher than in the pre-firing step and within the range of about 1100 C. to about 1450 C., and gradually cooling the fired product and thereby producing a magnetic composition containing metallic iron in crypto-crystalline form.

HENRY L. CROWLEY.

REFERENCES CITED The following references are of record in the file of this patent:

Kato et al. Apr. 9, 1935 Brill et a1. Nov. 14, 1939 Number Certificate of Correction Patent N 0. 2,575,099 November 13, 1951 HENRY L. CROWLEY It is hereby certified that error appears in the printed specification of the above numbered patent requiring, correction as follows:

Column 2, line 55, for contact read content; column 5, line 12, for modified read modifier; column 12, line 12, for or read of; column 16, Table 1, third column thereof, last line, for 586 read 585; column 17, line 41, for compaced read compacted; line 64, for intial read initial; column 20, line 45, before which insert of line 53, for incorporationg read incorporating; column 21, line 8, for iron second occurrence, read form; and that the said Letters Patent should be read as corrected above, so that the same may conform to the record of the case in theRatent Oflice.

Signed and sealed this 19th day of February, A. D. 1952.

THOMAS F. MURPHY,

Assistant Uommim'oner of Patcatc. 

1. AS A NEW ARTICLE OF COMMERCE, A MAGNETIC COMPOSITION POSSESSING USEFUL MAGNETIC AND ELECTRICAL PROPERTIES AND COMPOSED PRINCIPALLY OF FERRO-MAGNETIC MATERIAL CONTAINING A MIXED FERRITE AND METALLIC IRON IN CRYPTO-CRYSTALLINE FORM WHILE SUBSTANTIALLY FREE OF LARGE DISCRETE PARTICLES OF METALLIC IRON. 